As previously mentioned, in an embodiment of the invention the treated filler can be used in the manufacture of battery separators. Battery separators are microporous sheets that can be inserted between oppositely charged electrode plates in a lead/sulfuric acid battery. These microporous separators can prevent direct contact of the oppositely charged electrode plates and have sufficient porosity to allow ionic conductivity through the electrolyte (low electrical resistance). The separator should have sufficient puncture strength to prevent the creation of holes via punctures from sharp edges of other battery elements such as grids. Holes in a separator can lead to direct contact with time. Lowering the electrical resistivity and/or reducing the risk of punctured holes in the battery separator between the electrode plates can improve the reliability and flexibility in battery design and manufacture. Battery separator methods of manufacture are disclosed, for example, in U.S. Pat. Nos. 3,351,495 and 4,237,083.
In the production of polymeric compositions, it is common to incorporate fillers to improve the physical properties of the polymeric composition. The surfaces of such fillers are often modified to increase the reactivity and consequently the two and three-dimensional coupling of the filler within the polymeric composition. It is conventional in the rubber industry to incorporate carbon black and other reinforcing fillers into natural and synthetic rubber to increase the physical properties of the cured rubber vulcanizate. Fillers used to reinforce such polymeric compositions include natural and synthetic fillers.
One of the principal non-black fillers used in the rubber industry is amorphous precipitated silica. This siliceous filler is used to impart improved tensile strength, tear resistance and abrasion resistance to the rubber vulcanizate. Silica fillers are also used in combination with carbon blacks to obtain maximum mileage in passenger vehicle tires and off-the-road tires, e.g., tires for mining and logging operations and for road-building equipment. Such applications have become well established. When used as the sole reinforcing filler, silica fillers that are not well dispersed and/or coupled in the rubber do not provide the overall improved performance obtained by the use of carbon blacks alone. This is observed most readily in rubber vulcanizes used for tires, e.g., tire treads.
Various coupling materials, e.g., titanates, zirconates and silanes, have been suggested for use with silica fillers when such fillers are incorporated into polymeric compositions, e.g., rubber, in order to improve the performance of the rubber vulcanizate. Among the various organosilane coupling materials suggested for such use are the mercaptoalkyltrialkoxysilanes, e.g., mercaptopropyltrimethoxysilane, and the bis(alkoxysilylalkyl)polysulfides, e.g., 3,3′-bis(triethoxy-silylpropyl)tetrasulfide. The use of appropriate amounts of such coupling materials in siliceous filler-reinforced synthetic rubbers can provide at least equivalent performance to carbon black-reinforced synthetic rubbers in several key physical properties such as 300% modulus, tensile strength and abrasion resistance.
The high cost of various organosilanes, the irritating odors associated with some of the materials, the time and energy to mix them into and react with the filler in rubber and the alcohol generated by some of the materials can deter the more general use of siliceous fillers as the principal reinforcing filler in large volume rubber applications.
One drawback in using alkoxysilanes as coupling materials for silica fillers is their tendency to produce off-gases. In particular, hydrolysis of the alkoxy group(s) can result in the release of alcohol. In some cases the alkoxysilane and silica filler can be separately added directly to the rubber composition. In other cases the alkxoysilane can be first added to a siliceous filler that can be subsequently added to the rubber composition. In either case the hydrolysis of the available alkoxy groups can result in the release of alcohol some of which can be retained in the surrounding elastomer matrix. The portion of the alcohol retained in the surrounding elastomer matrix can result in porous zones or blisters which can form surface defects in the resulting formed rubber articles and/or can impair the dimensional stability of treads during extrusion and tire building. As a result, a low tread strip drawing speed should be maintained to conform with specifications, which can result in a decrease in production and concomitant increase in costs. The portion of the alcohol not retained in the surrounding elastomer matrix can create issue regarding the release of volatile organic compounds (VOC). This evolution and off gassing of alcohol can continue through the life of a product manufactured from an elastomer compounded with alkoxysilane coupling materials.
Bis(alkoxysilylalkyl)-polysulfides sometimes are used in place of mercaptoalkyltrialkoxysilanes to substantially reduce or minimize the associated irritating odors and scorch issues. Preparation of silica filled rubber compositions using bis(alkoxysilylalkyl)-polysulfides generally are performed within narrow temperature operating ranges. The mixing temperature should be high enough to permit the silica-silane reaction to take place rapidly but low enough to substantially preclude an irreversible thermal degradation of the polysulfane function of the coupling material and premature curing (scorch) of the rubber mixture. These limitations can result in decreased production and increased expense to achieve the desired dispersion of the silica throughout the rubber matrix. These limitations also can result in the retention in the rubber mixture of unreacted alkoxysilyl groups that can be available to further react during subsequent stages which can result in an undesirable increase in the compound viscosity, and a shorter shelf life. Moreover, the continuing reaction in the compound can evolve more (unevaporated) alcohol, can result in the alcohol related issues discussed in the previous paragraph.